Payloads use the active transmission Tx and/or reception Rx antennas and use distributed power radiofrequency amplification sections MPA to flexibly distribute a plurality of transmission channels to a plurality of output beams.
It is still currently being sought to improve existing solutions for calibrating such radiofrequency multichannel subsystems, the calibration taking place while the satellite is operating, at particular times over the lifetime of said multichannel subsystems in which the performance in terms of amplitude and phase difference between channels will have drifted excessively with respect to a required need, due for example to a change in external temperature conditions that are applied to the multichannel subsystems or to the ageing of their internal components.
The underlying problem of calibrating such multichannel subsystems is that of performing accurate calibration of the multichannel subsystem while it is transmitting operational communications carriers, that is to say achieving the desired amplitude matching and phase matching of the channels with one another with sufficient accuracy, while at the same time ensuring that the ongoing communications are maintained and their transmission quality is upheld.
To this end and as is known, patent application US 2012/0280748 A1, forming a first document, describes a multiport distributed amplification device that is compensated in the presence of traffic.
According to a first preferred embodiment, the multiport distributed amplification device that is compensated in the presence of traffic is configured so as to inject an unmodulated reference signal that is transmitted on a single carrier frequency. The value of this carrier frequency is chosen such that the reference signal is introduced between two communication channels without altering the performance thereof.
According to a second embodiment and as a variant, the unmodulated reference signal transmitted on a single carrier of the first embodiment is replaced with a spread-spectrum sinusoidal reference signal spread by way of a spreading code. In this case, the device for measuring the spread signal includes, in addition to the components that are used by the module for measuring the unmodulated signal, a module for despreading the received signal on the basis of the spreading code that is used, which is supplied by a spreading code generator.
According to the multiport distributed amplification device of the first document, the position of the reference signal to be injected into the guard bands between the traffic carriers depends on the frequency plan of the traffic carriers that originates from the telecommunications missions planning, this lacking flexibility.
In addition, the proposed waveforms for the calibration or reference signal to be injected, such as described in the first document for the spread-spectrum calibration, that is to say that of an unmodulated simple sinusoidal signal CW or that of a sinusoidal signal, spread by way of a spreading code, do not allow the reference signal to be injected into a useful traffic carrier in all cases of configuration of the useful traffic carrier, for example a high-order modulation carrier that is more sensitive to noise or with a high signal-to-noise ratio, or in cases in which the calibration time has to be limited. Specifically, the required power of the calibration signal, which has to be low enough so as not to disrupt transmission of the useful carrier within which it is situated, in order to comply for example with the quasi error-free packet error rate performance criterion of 10−7 for modulated useful carriers coded using DVB-S2, does not make it possible to estimate inter-channel amplitude and phase disparities with sufficient accuracy.
As is known, patent application US 2014/0354355 A1, forming a second document, describes a multiport distributed amplification device MPA configured so as to maximize isolation between its outputs by implementing a calibration method.
The calibration method uses the traffic signal itself to calibrate the channels of the MPA, but does not use any dedicated calibration signal that has an adapted waveform and is injected into at least one of the channels of the MPA.
If traffic carriers are utilized through frequency reuse in the MPA, degradation of the signal-to-noise ratio of one of these carriers at output of the MPA, caused by aggregated isolation leakages of the other co-frequency carriers, may give rise to dispersive or even erroneous estimations of the inter-channel amplitude and phase differences, leading to an incorrect calibration.
Furthermore, upon each acquisition of the traffic signal on an output of the MPA, the corresponding traffic signal at input of the MPA has to be acquired simultaneously so as to be able to be compared or correlated with the traffic signal at output. The solution described in the second document thus requires the use of a number of analogue-to-digital converters (ADC) twice that which is necessary in a conventional calibration system, such as that described in the first document.
Thus, the calibration systems and methods described in the first and second documents exhibit defects or drawbacks linked to operational constraints, notably with respect to frequency planning of the traffic, or linked to inter-channel error estimation performance limitations.
A calibration method and system that perform accurate calibration of the multichannel subsystem while it is transmitting operational communications carriers and that avoid the drawbacks of the systems of the abovementioned first and second documents are sought. The method and the system should make it possible to obtain the desired amplitude matching and phase matching of the channels with one another with sufficient accuracy, while at the same time ensuring that the ongoing communications are maintained and their transmission quality is upheld.
The calibration of the multichannel subsystem should be able to be performed in narrowband, for example over 41 MHz in the L band, but also over a wide frequency band, for example over 2 GHz in the Ku band or over 2.9 GHz in the Ka band.